The FTSE 100 hit an all-time high on 25 February as a solution to the Greek crisis came a step closer.
The listing ended on 6949.63, up by 37.47, and dwarfing the previous record close of 6930.2 that was achieved on 30 December 1999.
Market analysts said the eurozone's acceptance of Greece's proposal for economic reforms helped boost the FTSE 100, along with improved markets across the pond.
Augustin Eden, an analyst at Accendo Markets, said: "The positive open comes as Greece and the Eurogroup cleared one of the final hurdles towards extending the bailout for another four months after the latter approved the list of reforms presented at the 11th hour on Monday night.
"US markets ended higher on Tuesday as both the Wall Street and S&P indices hit record highs following [Janet] Yellen's speech, where many investors saw a 'subtle change in emphasis' from the Fed chair indicating that it would remain 'patient' regarding a rate rise any time soon."
The FTSE 100 has jumped by 6% since the turn of the year.